Singlet oxygen production by flavylium cations

Synthetic flavylium cations have been extensively employed as surrogates for unravelling the pH-dependent chemistry of naturally occurring anthocyanin plant pigments, as well as for the development of strategies for the stabilization of their color in practical applications. Although a wide variety of synthetic flavylium cations were previously found to exhibit detectable phosphorescence in a rigid trifluoroacetic acid (TFA)-acidified isopropanol glass at 77 K, the question remained as to the possibility of significant formation of triplet states in room temperature fluid solution. Of the 14 different flavylium cations investigated here, only four sensitized the formation of significant yields of singlet oxygen in TFA-acidified acetonitrile, while the other ten produced no detectable singlet oxygen. Correlation of the results with the chemical structures and the corresponding photophysical properties of the lowest energy excited singlet state of the compounds suggests that the formation of excited triplet states of natural anthocyanins is probably negligible in vivo in plants.